Brilliant and Angular-Independent Photonic Balls Using High-Refractive-Index Inorganic-Polymer Hybrid Materials

In the modern era, structural color materials are regarded as safe and promising alternatives to colorants that contain harmful components. However, developing structural color materials that exhibit vivid colors with minimal angular dependence is crucial for their practical application. In this study, spherical colloidal crystals (photonic balls) with bright colors and effectively suppressed angular dependence are developed using monodisperse high refractive index CeO₂ particles. To fabricate these photonic balls, CeO₂@PDA particles are synthesized by coating CeO₂ particles with polydopamine (PDA), a black component. The light-absorbing PDA coating on each particle uniformly reduces the multiple scattering of light to form a black background, allowing the CeO₂@PDA photonic balls to exhibit brilliant structural colors. Compared to SiO₂ particles photonic balls, which are widely studied in previous research, CeO₂@PDA photonic balls have a significantly reduced angular dependence of structural color hue due to their composition of materials with a higher refractive index. Additionally, the CeO₂@PDA photonic balls are heat-treated in a nitrogen atmosphere, transforming the polymer component on the particle surface into a black carbonaceous material with a higher refractive index. This process further reduces the angular dependence of structure color hues observed from the photonic balls and improves color vibrancy.